Sound reproducing system



S G.l SEARS Dec. 13, 1938.

SOUND REPRODUC ING SYSTEM Filed Nov. 1o, 1956 2 sheets-sheet 1 Dec. 13, 1938. s. G. sEARs SOUND REPRODUCING SYSTEM Filed Nov. l0, 1936 2 Sheets-Sheet 2 y! NVENIOR.

Patented Dec. 13, 1938 PATENT OFFICE SOUND REPRQDUCING SYSTEM Sterling G. Sears, Garden City, N. Y., assigner to Dictograph Products Company, Inc., New York,

N. Y., a corporation of Delaware Application November 10, 1936, Serial No. 110,061

claims.

This invention'relates to sound reproducing systems, and more particularly to systems designed for the reproduction of sounds with a volume and quality adapted for perception by persons listening to the same through a damping transmitting medium.

More particularly, the invention relates to devices for reproducing sound that are adaptedto be used in conjunction with ordinary sound re-v producing systems, such as phonographs, radio receivers, and the like, whereby they meet the requirements of an individual listener hearing through selectively absorbent soundtransmitting media. u

The invention has for its object generally the provisionof a system of the character indicated which is economical and eilcient, is used in place of ordinary loudspeakers when one is listening through a damping transmitting medium and is adapted for reproducing sound so as to be perceived with natural tone quality.

A specific object of the invention is to provide a system of sound reproduction for use in conjunction with radio receivers, and the like, whereby each listener of a group may use a commonvsource of electric energy and listen to a program of his or her selection to the exclusion of the other listeners.

Another object is,to provide a device, pref- 80 erably in the form of an attachment for use in conjunction with sound reproducing systems that is, strong and durableand is adapted for producing sound with4 a desired propagation characteristie in a sound absorbent transmitting medium. Still another object is to provide a system for reproducing sound in which the distortion that results from transmission through an intervening medium, such Aas air loaded with damping agents, is compensated so that the sound perceived has normal qualities.

Other' objects ofthe invention will in part be obvious and will in part appear hereinafter.'

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, which will be exemplied in the construction hereinafter set forth and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had` to the follcwing detailed description taken in connection with the accompanying drawings, in which:

Fig. 1 is a view showing mainly diagrammatu ically a system of sound reproduction adapted for the use of a plurality of individual listeners in ac,- cordance With the invention;

Fig. 2 is an enlarged sectional view showing the construction of the sound reproducing or speaking devices employed in Fig. 1;

Fig. 3 is a diagrammatic view showing volumefrequency characteristics of a speaking device that isconstructed in accordance with the invention; and

Fig. 4 is a view, similar to Fig. 1, illustrating a 10 modified form of the invention.

It is often desirable to provide facilities Whereby individual listeners to radio programs, and the like, in a group in a room may listen to the program of his or her own selection tothe exclusion 15 of the others. To provide each listener with an individual radio receiver involves considerable expense. Moreover, Where ordinary loudspeakers are used, onelistener cannotentirely exclude the program from another instrument that is differ- 20 ent; such arrangements in consequence have heretofore been deemed impractical.

In the practice of the present invention, the sound is reproduced by a translating device arranged to conne the sound to a nite region of 25 space.' To this end a confined transmitting medium is employed in association with the device reproducing the sound from electric impulses.

- By such means, each listener in a group in a room may listen to the program of his or her selection 30 while employing'a common radio energy collect-- ing means. A sound reproducing or speaking device coupled to propagate sound in a conned transmitting medium for this purpose may be described as a quiet speaker to distinguish it from 3'5 ordinary loudspeakers, and a radio set when equipped with one or `:more such arrangementaja "quiet radio. The listener, of course, placeshis or her head or ear against the medium and the sound is'perceived without setting the air out- 40 side oi' the space occupied by the medium into acoustic vibration, except to a negligible extent.

A To achieve the desired eifects uponthe ear of a listener the coupling between the speaking device and the medium isv made 'to be relatively efficient, 45

hence a relatively extended vibrating surface or diaphragm in the sound reproducing device is employed. Also it is advantageous to drive the diaphragm reactively. For this purpose a speaking device of the bone conduction type, that is employed as a hearing aidfor the hard of hearing and which is reactively driven, may be em ployed. Such devices are relatively small and durable and are readilyadapted for the driving of an extended surface in association with a con- 66 fined transmitting medium. Also to avoid undue fatigue of the listener it is desirable to have the sound transmitted by the medium reproduced at the ear of thelistener with a substantially normal quality, i. e., with a so-called flat volumefrequency characteristic.

It has been ascertained that, when a confined transmitting medium is associated with a speaking device of the character indicated, there is produced attenuation-and absorption of certain frequencies ln the sound transmitted due to the damping effect of the material constituting the medium. Where the conned medium is a pillow filled with feathers, air is in fact the transmitting medium but the feathers operate to load the medium with damping agents. The medium in such case is a composite medium with a non-.uniforml i 'attenuation and selective frequency absorption .pled amplifiers ||1 and Ilz.

characteristic. The sound therefore transmitted to a person whose head is on the pillow from a speaking device reproducing sound with a normal flat characteristics will be lacking those frequencies or bands which have been absorbed and will not be perceived as sound with the normal quality desired.

To achieve the desiredy effect upon listeners, in accordance with the present invention, means 'are associatedwith the speaking device that is coupled' to the transmitting medium which accentuate those frequencies selectively absorbed in the medium whereby the absorption is compensated for, and the sound perceived by the person listening has normal qualities.

Referring now to the drawings, and particularly .to Fig. l, I0 denotes a means, for example, an-

antenna, for collecting primary electric energy corresponding to the sound to be reproduced. This source of energy is shown as provided with twocoupled coils |01 and |02 so that two separate sets of impulses may be selected by tuning and, when suitably amplified, reproduced by means of quiet speakers in the one room. (It is obvious that the invention is not limited to the use oi two coils, any number that can be conveniently coupled electrically being employable.) The coils |01 and |02 are here shown with cou- Output coils |21 and |22 are connected to these amplifiers andpreferably have inductively coupled coils |31 and |32 for supplying a common loudspeaker I4. This latter is connected to coils |31 and |32 selectively by two-throw switches |51 and |51. Each of switches |51 and |511 has its other pole respectively connected through conductors leading to plug-in devices |61 and |62, from which lead conductors |11 and |12 to quiet speaking devices R1 and R2 respectively. For certain uses, for example, in hospitals where the use of a loudspeaker is undesirable, it is preferable to arrange the changeover switches |51 and |52to have the form of a holddown or momentary contact variety so that the loudspeaker is in circuit only while the user is tuning the set. Then upon completion of the tuning, the switch will immediately returnrto quiet position shown in the drawings. The loudspeaker may even be dispensed with, tuning being done with the quiet speaker. Each of these deviceshas acoustic coupling with a confined transmitting medium; the media being shown respectively at T1 and T2. Such media may have the same or individually different sound attenuating and frequency absorption characteristics.

Details of one form of a suitable speaking device, such as shown at R1 and R2 inV Fig. 1 and adapted for cooperative coupling with a, confined transmitting medium, are shown in Fig. 2. The arrangement illustrated is such that the device may readily be used in place of an ordinary loudspeaker with aV radio receiver. Here conductors supply the amplified electric impulses that represent sound and lead to a winding I8 which comprises the electromagnetic actuating means of the sound reproducing device. The winding i8 is disposed in a housing H which has an extended closure or diaphragm S which is adapted to be vibrated by reactive forces produced by the driving mechanism associated with winding i8 Within the housing H. 'I'nis mechanism is shown as a mechanical system composed of articulated members I9, 20, and,2|, which comprise a tuning fork, between the prong members I9 and 2 ifof which is disposed the winding i8. Todrive the diaphragm S, the prong member 2| is fixed to the diaphragm in any `convenient mann-er. To support the winding |8 in driving position, it is preferably mounted on a core portion depending from the free prong member i9, there being an air gap 22 of suitable width between, this portion and the prong member 2|. The transmitting medium which is interposed between the diaphragm S and the ear offthe person using the device is here Y shown 'at The sound yattenuating and frequency absorption characteristics of this medium are shown in Fig. 3.

In Fig. 3, the volume of sound reproduced is denoted by ordinates, while frequency is denoted by abscissae; the values of the frequency increasing from left to right. The curve u shows the normal volume-frequency characteristic of the sound produced by any vibrating surface. when actuated to produce sound adapted to be perceived through normal air conduction by a person of normal hearing. This would also be the characteristicof the sound reaching-the brain of a person hard of hearing when using a boneconduction device suited to his particular type of deafness. When an absorptive transmitting medium, such as indicated at T, intervenes, the se lective absorption takes place in the higher frequencies. The propagation characteristic of the medium is indicated by the curve 0,' which is shown as starting at a point close to the curve a in the low frequency region, i. e. in the'region of cycles per second; the curve c thereafter dropping away from the curve u' as the values of the frequency increase, the departure ,being relatively great in the high frequency region, i. e. above 1,000 cycles per second. This is due to the fact that for the medium comprising air loaded with feathers, taken by way of example, the absorption of sound increases as the frequency increases. In Fig. 3 the frequencies are shown on a logarithmic scale.

To compensate for the absorption characteristic indicated by' curve v, the quiet speaker of the present invention is made to reproduce sound with an accentuated or augmented volume for those frequencies which are selectively absorbed,

i. e., the speaker is made to reproduce sound with a response characteristic which is here depicted as curve w. Here, it is seen that the volume of sound reproduced by the surface S is increased for frequencies between 2,000 and 6,000 cycles per second and is above normal so that the ab' 4sorption introduced by transmission through the medium 'I' brings the volume perceived by the person, hearing through medium T, down to the level depicted in the Vperception characteristic curve u1.

Thus, the distortion produced by the attenuat- The desired compensating effects to be introlaltered and given desired values.

duced into the sound reproduced by the speaking device of the present invention are accomplished either mechanically or electrically,

To introduce` compensating effects mechanically into a quiet speaking device, the factors which control the vibration and resonance .characteristics thereof are altered, for example, the stiffness of-the yoke or reed member 20 may be The ratio of high frequency to low frequency may be increased by increasing the stiffness in any suitable manner, for example, by altering the cross-section of the reed or yoke member 20 to increase its thickness. Also it will be understood that the character and area of the sound radiating surface of diaphragm S may be altered in order to achieve a desired modification in the response characteristic, for example, the area of diaphragm S may be increased to increase the ratio of low frequency to high frequency energy in the sound reproduced.

To provide the desired compensating effects electrically instead of mechanically, a speaking device of the type shown at R1 and Rz but having substantially no resonance (i. e., none with appreclable effect), is employed and arranged to be actuated by a modified electric circuit, such as shown in Fig. 4. Here, a common antenna lli supplies two sets of amplifying stages H1 and liz, as in Fig. 1. Each of these stages has an output circuit and a two-throw switch, as shown at |51 and 152 in series therewith. One pole of each of these switches is connected to a common output coil l2 that is the yprimary of a transformer which has a secondary I3 for supplying a common loudspeaker at Mi. 'I'he other poles of each of switches 51 and 52 have connections unavoidably introduced` by a coupled conned transmitting medium. The response' characteristic curve w which d picts that desired for a speaking device CQQDEIatively associated with a confined transmitting medium when loaded with damping agents, such als feathers, resembles the response characteristic that it is desired to impart to a speaking device with an extended diaphragm for the propagation of sound in ordinary air, disclosed in`my copending application, Serial No. 114,621, led December 7, 1936. The device disclosed in myv said copending application is of/t'he general typ-e here employed and is adapted for use by a person of normal hearing listening in a room and perceives sound with a normal flat characteristic. Also, a person hard of hearing and who belongs to the class whose perception of high frequencies is leading respectively to primaries of transformers Ving matched impedances, is accomplished by introducing suitable elements of impedance to achieve a response characteristic in accordance with the curve w. This modication may be accomplished in any one of several ways. For example, elements of inductance as shown at L1 and Lz may be disposed in shunt with the coils 281 and 282 and elements of capacitance K1 andK2 inserted in series therewith, the values of which are chosen so that sound is reproduced by the diaphragm ofthe coupled speaking device with a characteristic shown by the curve urin Fig, 3.

The mode of operation of the speaking devices shown in Figs, 1 and 4 is believed to be readily understood from the above description beinvention being dependent on the nature of the transmitting medium employed, the curvew, here illustrated by way of example, being that which gives the compensating response for the characteristic curve v, which depicts the propagation characteristic for a pillow filled with feathers. In the event that a different transmitting medium were used the attenuation and absorption characteristic would in general givea curve other than that depicted at o. To produce the desired effect at the ear of the listener, the distortion, whatever its nature and character, is corrected by the speaking device having a compensating response characteristic, employed in accordance with the invention.

Since certain changes may be made in the above construction and different embodiments of the invention couldbe made Without departing from the scope thereof, it is intended Athat all matter contained in the above description or shown in the accompanying drawings shall be in terpreted as illustrative and not in a limiting sense,

Having described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In a sound reproducing system, the cornbination with a 'transmitting medium through which a person desiring to listen may receive sound and which is of a character that attenuates sound and absorbs frequencies throughout the normal hearing range in a non-uniform manner,

of a primary source of electric impulses representing sounds-to be reproduced, means in association with said source for amplifying said impulses to values representing sounds of a quality and amplitude adapted for normal reproduction, an extended surface adapted to be placed in sound propagating contact with said medium, a mechanical system for vibrating said surfacereactively, means for driving said system' actuated electrically by said amplified impulses, and means associated with said driving means for modifying the resonance characteristic thereof and accentuating the vibrating frequencies corresponding to the frequencies absorbed by said medium.

2. In a sound reproducing system, the combination with a transmitting medium through which a person desiring to listen may receive sound and which is of a character that attenu-A ates sound and absorbs frequencies throughout the normal hearing. range in a non-uniform manner, of a primary source of electric impulses representing sounds to be reproduced, means in association with said source for amplifying said an extended surface adapted to be placed in sound propagating contact with said medium, av

mechanical system of members connected for vibrating said surface reactively, means for driving said system actuated electrically by said amplied impulses, the moving members of said system having a predetermined mass and stiffness so as to alter their response characteristic from that intended for normal sound reproduction to one in which selected frequencies are accentuated to a degree which compensates for the frequencies absorbed by said medium.

3. In a sound reproducing system, the combination with a transmitting, medium through which a person desiring to listen may receive sound and which is of a character that attenuates sound and absorbs frequencies throughout the normal hearing range in a non-uniform manner, of a. primary source of electric impulses representing sounds to'be reproduced, means in association with said source for'amplifying said impulses to values representing sounds of a quality and amplitude adapted for normal reproduction, an ex-v tended surface adapted to be placed in sound propagating contact with said medium, a rnechanical system of articulated members connected for vibrating said surface reactively, means for driving said system actuated electrically by said amplified impulses including an electric circuit with balanced impedances, and means comprising elements of impedance inserted in said circuit for modifying the resonance characteristic thereof electrically and accomplishing the accentuation of vibrating frequencies corresponding to the frequencies -absorbed by said medium.

4. In a sound reproducing system, the combination with a damping transmitting medium in a coniined space and adapted for conveying sound to an individual listener, of a translating .device having an extended diaphragm in sound propagating contact with said medium, a vibrating mechanism floatingly coupled to said diaphragm for actuating it reactively, and means for supplying electric energy for driving said mechanism in accordance with sound to be reproduced.

5. In a sound reproducing system, the combination With a damping transmitting medium in a confined space and adapted for conveying sound to an individual listener, of a translating device having an extended diaphragm in sound propagating contact with said medium, a vibrating mechanism comprising articulated members disposed to form a tuning fork and coupled floatingly to actuate said diaphragm reactively, certain of said members being stiffened, a mass-member of appreciable Weight attached to a floating member of said mechanism, and means for supplying electric energy for driving said mechanism in accordance with sound to be reproduced.

STERLING G. SEARS.

g ...fm 

